No immediate nuclear war risk: The treaty was about limiting deployed warheads and providing inspections, not a peace treaty.
No immediate arms race: Russia lacks the financial resources to compete in an arms race.
Russia's interest: Russian officials have a better understanding of the issue than the Trump administration, and Russia cannot afford another arms race.
US nukes: The US is unlikely to increase its overall number of nukes, but more stored nukes might be deployed and aimed.
Why it matters: The expiration of the treaty undermines global nuclear arms control efforts. It removes the precedent for other countries, like China, to engage in nuclear arms control, and gives non-nuclear countries less reason not to pursue nuclear weapons.
Long-term implications: Nuclear arms are strategic and shape the future world. The Trump administration's attitude of "If it expires, it expires" has made the US and the entire world less safe. This issue will need to be addressed by a future president